9 that dark line that runs from your pubic bone straight up the center of your belly known as the linea nigra is said to indicate babys sex if the line stops at your belly button youre having a girl if the line continues up to your ribs and chest its a boy. Yes choosing the sex of your child is technically possible thanks to advances in fertility treatments that allow doctors to identify male and female embryos sex selection is an option for couples who want to avoid passing sex linked genetic disorders to their children. Molly cerreta smith we all learned in science class that men determine the sex of a baby based on whether their sperm is carrying an x or a y chromosome combined with the mothers x chromosome an x chromosome will produce a baby girl xx and a y chromosome will make a baby boy xy end of story right. In a study published in the journal birth in september 1999 johns hopkins school of public health researchers asked 104 pregnant women to guess their babys sex using whatever method they liked. There are several ways you can find out your babys sex starting at 10 weeks of pregnancy if you have noninvasive prenatal testing nipt a blood test that can detect down syndrome and a few other chromosomal conditions it takes a week or two to get the results